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The solidus was introduced by Diocletian in AD 301 as a replacement of the aureus, composed of relatively solid gold and minted 60 to the Roman pound. His minting was on a small scale, however, and the coin only entered widespread circulation under Constantine I after AD 312, when it permanently replaced the aureus.[1] Constantine's solidus was struck at a rate of 72 to a Roman pound (of about 326.6 g) of pure gold; each coin weighed 24 Greco-Roman carats(189 mg each),[2] or about 4.5 grams of gold per coin. By this time, the solidus was worth 275,000 increasingly debased denarii. With the exception of the early issues of Constantine the Great and the odd usurpers the Solidus today is a much more affordable Gold Roman Coin to collect compared to the Older Aureus. Especially those of Valens Honorius and later Byzantine issues. The solidus was maintained essentially unaltered in weight, dimensions and purity until the 10th century. During the 6th and 7th centuries "lightweight" solidi of 20, 22 or 23 siliquae (one siliqua was 1/24 of a solidus) were struck along with the standard weight issues, presumably for trade purposes or to pay tribute. Many of these lightweight coins have been found in Europe, Russia and Georgia. The lightweight solidi were distinguished by different markings on the coin, usually in the exergue for the 20 and 22 siliquae coins and by stars in the field for the 23 siliquae coins.

Solidus of Constantine I, minted in AD 324 or 325

In theory the solidus was struck from pure gold, but because of the limits of refining techniques, in practice the coins were often about 23k fine (95.8% gold). In the Greek-speaking world during the Roman period, and then in the Byzantine economy, the solidus was known as the νόμισμα nomisma (plural nomismata).[2] In the 10th century Emperor Nicephorus II Phocas (963–969) introduced a new lightweight gold coin called the tetarteron nomisma that circulated alongside the solidus, and from that time the solidus (nomisma) became known as the ἱστάμενον νόμισμα histamenon nomisma in the Greek speaking world. Initially it was difficult to distinguish the two coins, as they had the same design, dimensions and purity, and there were no marks of value to distinguish the denominations. The only difference was the weight. The tetarteron nomisma was a lighter coin, about 4.05 grams, but the histamenon nomisma maintained the traditional weight of 4.5 grams. To eliminate confusion between the two, from the reign of Basil II (975–1025) the solidus (histamenon nomisma) was struck as a thinner coin with a larger diameter, but with the same weight and purity as before. From the middle of the 11th century the larger diameter histamenon nomisma was struck on a concave flan, though the smaller tetarteron nomisma continued to be struck on a smaller flat flan.

Former money changer Michael IV the Paphlagonian (1034–41) assumed the throne of Byzantium in 1034 and began the slow process of debasing both the tetarteron nomisma and the histamenon nomisma. The debasement was gradual at first, but then accelerated rapidly: about 21 carats (87.5% pure) during the reign of Constantine IX (1042–1055), 18 carats (75%) under Constantine X (1059–1067), 16 carats (66.7%) under Romanus IV (1068–1071), 14 carats (58%) under Michael VII (1071–1078), 8 carats (33%) under Nicephorus III (1078–1081) and 0 to 8 carats during the first eleven years of the reign of Alexius I (1081–1118). Alexius reformed the coinage in 1092 and eliminated the solidus (histamenon nomisma) altogether. In its place he introduced a new gold coin called the hyperpyron nomisma at about 20.5k fine (85%). The weight, dimensions and purity of the hyperpyron nomisma remained stable until the fall of Constantinople to the Crusaders in 1204. After that time the exiled Empire of Nicea continued to strike a debased hyperpyron nomisma. Michael VIII recaptured Constantinople in 1261, and the Byzantine Empire continued to strike the debased hyperpyron nomisma until the joint reign of John V and John VI (1347–1354). After that time the hyperpyron nomisma continued as a unit of account, but it was no longer struck in gold.

From the 4th to the 11th centuries, solidi were minted mostly at the Constantinopolitan Mint, but also in Thessalonica, Trier, Rome, Milan, Ravenna, Syracuse, Alexandria, Carthage, Jerusalem and other cities. During the 8th and 9th centuries the Syracuse mint produced a large number of solidi that failed to meet the specifications of the coins produced by the imperial mint in Constantinople. The Syracuse solidi were generally lighter (about 3.8g) and only 19k fine (79% pure).

Although imperial law forbade merchants from exporting solidi outside imperial territory, many solidi have been found in Russia, Central Europe, Georgia, and Syria. In the 7th century they became a desirable circulating currency in Arabian countries. Since the solidi circulating outside the empire were not used to pay taxes to the emperor, they did not get reminted, and the soft pure-gold coins quickly became worn.[2]

Through the end of the 7th century, Arabian copies of solidi — dinars minted by the caliph Abd al-Malik ibn Marwan, who had access to supplies of gold from the upper Nile — began to circulate in areas outside the Byzantine Empire. These corresponded in weight to only 20 carats (4.0 g), but matched the weight of the lightweight (20 siliquae) solidi that were circulating in those areas. The two coins circulated together in these areas for a time.[2]

The solidus was not marked with any face value throughout its seven-century manufacture and circulation[citation needed]. Fractions of the solidus known as semissis (half-solidi) and tremissis (one-third solidi) were also produced.[citation needed]

The word soldier is ultimately derived from solidus, referring to the solidi with which soldiers were paid.[3]

In medieval Europe, where the only coin in circulation was the silver penny (denier), the solidus was used as a unit of account equal to 12 deniers. Variations on the word solidus in the local language gave rise to a number of currency units:

In the French language, which evolved directly from common or vulgar Latin over the centuries, solidus changed to soldus, then solt, then sol and finally sou. No gold solidi were minted after the Carolingians adopted the silver standard; thenceforward the solidus or sol was a paper accounting unit equivalent to one-twentieth of a pound (librum or livre) of silver and divided into 12 denarii or deniers.[4] The monetary unit disappeared with decimalisation and introduction of the Franc during the French revolution (1st republic) in 1795, but the coin of 5 centimes, the twentieth part of the Franc, inherited the name "sou" as a nickname: in the first half of the 20th century, a coin or an amount of 5 francs was still often referred to as cent sous.

To this day, in French around the world, solde means the balance of an account or invoice, and is the specific name of a soldier's salary. Although the sou as a coin disappeared more than two centuries ago, the word is still used as a synonym of money in many French phrases: avoir des sous is being rich, être sans le sou is being poor (same construction as "pennyless").

In Canadian French, sou and sou noir are commonly employed terms for the Canadian cent. Cenne and cenne noire are also regularly used. The standard French word centime is not used in Quebec. In Canada one hundredth of a dollar is officially known as a cent (pronounced /sɛnt/) in both English and French. However, in practice, a feminine form of cent, cenne (pronounced /sɛn/) has completely replaced the official "cent". Spoken and written use of the official masculine form of cent in francophone Canada is exceptionally uncommon. Interestingly, quarter dollars in colloquial Quebec French are sometimes still called trente-sous (thirty cents), because of a series of changes in terminology, currencies, and exchange rates. After the British conquest of Canada in 1759, French coins gradually fell out of use, and sou became a nickname for the halfpenny, which was similar in value to the French sou. Spanish pesos and U.S. dollars were also in use, and from 1841 to 1858 the exchange rate was fixed at $4 = £1 (or 400¢ = 240d). This made 25¢ equal to 15d, or 30 halfpence i.e. trente sous. In 1858, pounds, shillings, and pence were abolished in favour of dollars and cents, and the nickname sou began to be used for the 1¢ coin, but the misnomer un trente-sous for a 25¢ coin has endured.[5] In the vernacular Quebec French sous and cennes are also frequently used to refer to money in general, especially small amounts.

The name of the medieval Italian silver soldo (plural soldi), coined since the 11th century, was derived from solidus.

This word is still in common use today in Italy in its plural soldi with the same meaning as the English equivalent "money". The word "saldo", like the French solde mentioned above, means the balance of an account or invoice. It also means "seasonal rebate", probably by contamination between the original meaning and the English word "sales".

As with soldier in English, the Spanish and Portuguese equivalent is soldado (almost the same pronunciation). The name of the medieval Spanishsueldo and Portuguese soldo (which also means salary) were derived from solidus; the Filipino term sweldo is derived from the Spanish.

The Spanish and Portuguese word saldo, like the French solde, means the balance of an account or invoice. It is also used in some other languages, such as German and Afrikaans.

Some have suggested that the Peruvian unit of currency, the sol, is derived from solidus, but the standard unit of Peruvian currency was the real until 1863. Throughout the Spanish world the dollar equivalent was 8 reales ("pieces of eight"), which circulated legally in the United States until 1857. We hear echoes of that time in the expression "two bits" for a quarter dollar, and the real was last used for accounting in the US stock market, which traded in 1/8 dollars until 2001.

The Peruvian sol was introduced at a rate of 5.25 per British Pound, or just under four shillings (the legacy soldus). The term soles de oro was introduced in 1933, three years after Peru had actually abandoned the gold standard. In 1985 the Peruvian sol was replaced at one thousand to one by the inti, representing the sun god of the Incas. By 1991 it had to be replaced with a new sol at a million to one, after which it remained reasonably stable.

King Offa of Mercia began minting silver pennies on the Carolingian system c. 785. As on the continent, English coinage was restricted for centuries to the penny, while the scilling, understood to be the value of a cow in Kent or a sheep elsewhere,[6] was merely a unit of account equivalent to 12 pence. The Tudors minted the first shilling coins. Prior to decimalization in the United Kingdom in 1971, the abbreviation s. (from solidus) was used to represent shillings, just as d. (denarius) and £ (libra) were used to represent pence and pounds respectively.

Under the influence of the old long S ⟨ſ⟩,[citation needed] the abbreviations "£sd" eventually developed into the use of a slash ⟨/⟩, which gave rise to that symbol's ISO and Unicode name "solidus".

Porteous, John (1969). "The Imperial Foundations". Coins in history : a survey of coinage from the reform of Diocletian to the Latin Monetary Union. Weidenfeld and Nicolson. pp. 14–33. ISBN0-297-17854-7.